I've had both running on the same computer, which is how I did my testing of the beta. I had the same accounts setup as well and no problems. When I am at home, I have the option of using VPN for mail if I choose. You shouldn't have a problem using Outlook for work email and ZDesktop for everything else....it will be running 2 programs, and so more computer resources, but on most modern systems, that's probably not going to cause you any pain.
1+ Year on Zimbra
Release 6.0.6_GA_2330.RHEL5_64_20100505193959 RHEL5_64 NETWORK edition.
RHEL 5.3 - After complete and utter Ubuntu 8.0.4 failure.
AMD Dual Core 2216 x2
16GB of RAM
425 GB (6 disks) of RAID 10 storage
3ware + Seagate SATAs
Virtual Server as emergency system using rsync.